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06-09-2015

World’s Smallest Weld in Thermoplastic Material

EB lithography and diode laser welding – key to achieving the world's smallest welds in plastics (figure: TWI)

A novel welding technique enabled by electron beam lithography involving laser welding along absorber dye patterns has been developed by TWI Ltd., Cambridge, UK, in collaboration with the University of Cambridge. The result is the world’s smallest weld in thermoplastic material, bringing with it new applications for smaller scale biological analysis chips, chemical micro-reactors and electronics products.

As industry seeks to produce even smaller scale complex plastic components, there is a call for new research to allow welds in plastics of 10 μm and below. Until recently, the smallest laser welds possible measured between 10 and 20 μm in width. However, this is at an infrared laser’s resolution limit, and smaller welds are not possible using control of the laser focus spot size alone.

It was work carried out by TWI and the University of Cambridge’s Cavendish Laboratory into precise patterning of laser absorber dye on a plastic surface to define weld position, that sparked off the latest advance. Having proved that the process using laser absorber dye resist material could allow joints to be made between plastics, the project team studied the use of electron beam lithography to pattern the absorber and enable welds with a width smaller than 10 μm, mimicking methods used to build microelectronic circuits. The challenge was to generate micro-channels and infrared absorber tracks at their edges simultaneously, and to seal the channels.

Welds in PMMA, 0.5 microns wide at 5 micron intervals (figure: TWI)

Welds in PMMA, 0.5 microns wide at 5 micron intervals (figure: TWI)

With polymethyl methacrylate (PMMA) thermoplastic as a base material, the team carried out trials, following the principle of transmission laser welding using a thin coating of infrared absorbent material at the joint interface. The coating was patterned using electron beam lithography to the required resolution in a reproducible manner, so it could be retained after welding. Joint strength was ratified using larger-scale samples.

The trials culminated in successful demonstration of a series of laser welded joints with widths of 1 μm and channels of 5 μm. Smaller welds with 0.5 μm were also demonstrated leading to the conclusion that it is possible to make plastics components with a high density of structure and resolution below 1 μm, and that welding can be applied without excessively heating regions outside the weld lines.

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